The present invention relates to frequency converters and, more particularly, to a frequency converter which does not introduce unwanted harmonics, which is capable of driving a filter having a limited input signal handling capability, and which is unaffected by the parameters of active circuit elements.
Frequency converter circuits are often required to operate in the presence of one or more unfavorable conditions which fully test the mettle of the circuit designer. In some applications, however, circuit design virtually becomes enigmatic. One such application is the measurement of intermodulation distortion introduced in a communications channel. In a particular intermodulation distortion test set described herein, a frequency converter is required to provide an intermediate frequency (IF) signal in response to discrete input tones which are to be mixed with a local oscillator frequency residing very close to the output IF. The conditions imposed upon the frequency converter by the system are: (1) the output IF signal must not exceed -10 dbm in order not to overdrive the output IF filter; (2) the circuit gain must be independent of the parameters of active elements in the circuit; (3) the signal frequencies are much lower than the output IF requiring that the local oscillator frequency be relatively close to the IF and therefore presenting the danger that the converter will be overdriven; and (4) the possibility of generating undesired harmonics of the input signal must be eliminated.
It is an object of the present invention to provide a frequency converter capable of operating within the framework of th foregoing enumerated conditions.